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    • Decreasing the uncertainty of atomic clocks via real-time noise distinguish
    • The environmental perturbation on atoms is a key factor restricting the performance of atomic frequency standards, especially in the long-term scale. In this Letter, we perform a real-time noise distinguish(RTND) to an atomic clock to decrease the uncertainty of the atomic clock beyond the level that is attained by the current controlling method. In RTND, the related parameters of the clock are monitored in real time by using the calibrated sensors, and their effects on the clock frequency are calculated. By subtracting the effects from the error signal, the local oscillator is treated as equivalently locked to the unperturbed atomic levels. In order to perform quantitative tests, we engineer time-varying noise much larger than the intrinsic noise in our fountain atomic clock. By using RTND, the influences of the added noises are detected and subtracted precisely from the error signals before feeding back to the reference oscillator. The result shows that the statistical uncertainty of our fountain clock is improved by an order of magnitude to 2 × 10~(-15). Besides, the frequency offset introduced by the noise is also corrected, while the systematic uncertainty is unaffected.
    • Parity chain and parity chain breaking in the two-level cavity quantum electrodynamics system
    • We investigate the transitions between energy levels and parity symmetry in an effective two-level polar molecule system strongly coupled with a quantized harmonic oscillator. By the dressed-state perturbation theory,the transition diagrams between the dressed-state energy levels are presented clearly and show that the odd(even) parity symmetry is broken by the permanent dipole moment(PDM) of the polar molecules. By the analytical and numerical methods, we find that when the coupling strength and the PDM increase, the more frequency components are induced by the counter-rotating terms and PDM.
    • Two-color cesium magneto-optical trap with 6S1/2-6P3/2-7S1/2(852 nm + 1470 nm) ladder-type system
    • A 1470 nm+ 852 nm two-color(TC) cesium(Cs) magneto-optical trap(MOT) with a 6S_(1/2)-6P_(3/2)-7S_(1/2) laddertype system is proposed and experimentally investigated. To the best of our knowledge, it is the first report about the 1470 nm + 852 nm Cs TC-MOT. One of the three pairs of the 852 nm cooling and trapping beams(CTBs) in a conventional Cs MOT is replaced with a pair of the 1470 nm CTBs. Thus, the TC-MOT partially employs the optical radiation forces from photon scattering of the 6P_(3∕2)(F'= 5)-7S_(1∕2)(F''= 4) excited-state transition(1470 nm). This TC-MOT can cool and trap Cs atoms on both the red-and blue-detuning sides of the twophoton resonance. This work may have applications in cooling and trapping of atoms using inconvenient wavelengths and background-free detection of cold and trapped Cs atoms.
    • Artificial neural-network-based visible light positioning algorithm with a diffuse optical channel
    • Visible light positioning becomes popular recently. However, its performance is degraded by the indoor diffuse optical channel. An artificial neural-network-based visible light positioning algorithm is proposed in this Letter,and a trained neural network is used to achieve positioning with a diffuse channel. Simulations are made to evaluate the proposed positioning algorithm. Results show that the average positioning error is reduced about 13 times, and the positioning time is reduced about two magnitudes. Moreover, the proposed algorithm is robust with a different field-of-view of the receiver and the reflectivity of the wall, which is suitable for various positioning applications.
    • Symbol error rate performance analysis of soft-decision decoded MPPM free space optical system over exponentiated Weibull fading channels
    • The symbol error rate(SER) performance of a multipulse pulse-position modulation(MPPM) free space optical(FSO) system under the combined effect of turbulence-induced fading modeled by exponentiated Weibull(EW)distribution and pointing errors with a soft-decision detector is investigated systematically. Particularly, the theoretical conditional SER(CSER) of soft-decision decoded MPPM is derived. The corresponding closed-form CSER is obtained via curve fitting with the Levenberg–Marquardt method. The analytical SER expression over the aggregated fading channels is then achieved in terms of Laguerre integration. Monte Carlo simulation results are also offered to corroborate the validity of the proposed SER model.
    • Highly linear W-band receiver front-end based on higher-order optical sideband processing
    • A highly linear W-band receiver front-end based on higher-order optical sideband(OSB) processing is proposed and experimentally demonstrated. Two-tone analysis shows that by manipulating higher-order OSBs, the thirdorder intermodulation distortion(IMD3) introduced by optoelectronic components(mainly modulators) in the receiver front-end can be further suppressed, and a 9 dB improvement of the ratio of the fundamental and IMD3 can be attained. In the experiment, the spurious-free dynamic range of the W-band receiver front-end is up to 122.1 d B·Hz~(2∕3), with improvement by 9 dB compared with that of only processing the five OSBs.
    • Experimental demonstration of indoor interfered visible light communication system employing successive interference cancellation
    • Multipath interference induced power fading occurs when the transmission path lengths from the light emitting diodes to a single receiver are different in a visible light communication system. To solve this problem, we apply a QR-decomposition-based channel equalizer(QR-CE) to achieve successive interference cancellation for a discrete Fourier transform spreading(DFT-S) orthogonal frequency division multiplexing(OFDM) signal.We experimentally demonstrate a 200 Mb/s DFT-S OFDM over a 2 m free-space transmission. The experimental results show that a DFT-S OFDM with QR-CE attains much better bit error rate performance than a DFT-S OFDM with conventional CEs. The impacts of several parameters on a QR-CE are also investigated.
    • Automated segmentation and quantitative study of retinal pigment epithelium cells for photoacoustic microscopy imaging
    • We develop an improved region growing method to realize automatic retinal pigment epithelium(RPE) cell segmentation for photoacoustic microscopy(PAM) imaging. The minimum bounding rectangle of the segmented region is used in this method to dynamically update the growing threshold for optimal segmentation. Phantom images and PAM imaging results of normal porcine RPE are applied to demonstrate the effectiveness of the segmentation. The method realizes accurate segmentation of RPE cells and also provides the basis for quantitative analysis of cell features such as cell area and component content, which can have potential applications in studying RPE cell functions for PAM imaging.
    • Laser-accelerated self-assembly of colloidal particles at the water-air interface
    • We experimentally demonstrate that optical tweezers can be used to accelerate the self-assembly of colloidal particles at a water–air interface in this Letter. The thermal flow induced by optical tweezers dominates the growth acceleration at the interface. Furthermore, optical tweezers are used to create a local growth peak at the growing front, which is used to study the preferential incorporation positions of incoming particles.The results show that the particles surfed with a strong Marangoni flow tend to fill the gap and smoothen the steep peaks. When the peak is smooth, the incoming particles incorporate the crystal homogeneously at the growing front.
    • Investigation of ellipticity and pump power in a passively mode-locked fiber laser using the nonlinear polarization rotation technique
    • An elliptical initial polarization state is essential for generating mode-locked pulses using the nonlinear polarization rotation technique. In this work, the relationship between the ellipticity ranges capable of maintaining mode-locked operation against different pump power levels is investigated. An increasing pump power, in conjunction with minor adjustments to the polarization controller's quarter waveplate, results in a wider ellipticity range that can accommodate mode-locked operation. Other parameters such as noise, pulsewidth, and average output power are also observed to vary as the ellipticity changes.